Canadian oil sands are a combination of clay, sand, water, and bitumen, heavy black viscous oil. Oil sand, as mined commercially, typically contains an average of 10-12% bitumen, 83-85% mineral matter and 4-6% water. The hot water extraction process is a common commercial process used for extracting bitumen from mined oil sands. Almost all of the water withdrawn for oil sand operations usually ends up in tailings ponds. Both primary and final extraction plant tailings are pumped to a retention pond for storage.
When these effluent streams containing bitumen, naphtha, water, and solids are discharged to the pond, a portion of the residual bitumen and diluents naphtha floats to the surface of the pond. The dense sand fraction present in the primary stream typically settles rapidly but the lighter solid fines suspension in water usually settles very slowly, forming a zone of sludge. After a period of settling a shallow layer of relatively clear water develops near the surface of the pond. Water from this layer is usually recycled to the extraction process. But the majority of water remains in this sludge, a water-bitumen-fine solids emulsion that is very difficult to break. The water-bitumen-fine solids emulsion needs to be broken to separate this sludge into the bitumen, fine solids, and water.
Refineries produce products from ever increasing number of different feed stocks, for example sweet crude, sour crude, asphaltic crude and aliphatic crude, and emulsion problems can develop with the solids, oil and water that find their way into the oil-water separator. The separator may produce a crude outlet stream of a high level of emulsion, which the emulsion being usually skimmed-off and moved into a holding tank. Usually, the water is moved from the holding tank to a sour water stripper and the oil is routed back to either a crude charge tank or directly to the feed into a crude unit on its way to desalters. These streams tend to put major stress on the desalters and in turn on main fractionators at the crude unit.
Another area of concern for breaking down emulsions in a refinery is at the level of the tank farm. Throughout the refinery, different process streams deposit solids, oil and water on the bottoms of the tanks in what is termed as sludge. This sludge is typically high in solids of mostly inorganic compounds. The oil and water are usually bound very tightly with these solids, forming an emulsion that does not allow for an easy separation. When the levels of this sludge become too high in the tanks, they must be cleaned. However, if the emulsion cannot be separated, it becomes hazardous waste that can only be disposed of at high cost.
Emulsions may also problematic in bilge water of big ships such as Navy ships. Bilge water is a collection of different streams that collect at the lowest point in the ship. The bilge water contaminants usually include oil, non-ionic detergents, commercial laundry detergents, cleaners, solvents, suspended solids and dissolved solids. Typical values for oil and grease content in the bilge water may range between 100 and 10,000 ppm, which is well above a typical discharge limit of e.g. 15 ppm. The oil and water are immiscible, and when sufficient mechanical energy is added to the mixture, a stable oil-in-water emulsion may form. The bilge water emulsions are composed of oil droplets dispersed in water and may also contain smaller droplets of the continuous water phase dispersed within each droplet of the dispersed oil phase, yielding what is known as a double emulsion. Among the contaminants, the nonionic detergents, commercial laundry detergents and cleaners are emulsifying agents. The mixture of these emulsifying agents is more effective than a single emulsifying agent in forming a complex at the interface between the oil and water, resulting in a low interfacial tension and a strong interfacial film (Schramm, L. L., Ed., “Emulsions Fundamentals and Applications in the Petroleum Industry”, Published by American Chemical Society, Washing, DC, 1992) around the oil droplets. Lowering the interfacial tension makes it easier to create small oil droplets that do not coalesce to form larger droplets. The presence of different emulsifying agents in the bilge water stabilizes these emulsions. Dissolved and suspended solids in bilge water also contribute to the stabilization of oil-in-water emulsions. In order to break the oil-in-water emulsions in the bilge water, the detergent or surfactant thin films around the oil droplets must be broken so that the oil droplets undergo coalescence.
Because of the strong, thin surfactant films around the oil droplets, gravitational separation is typically not practical for removing small droplets from bilge water in a reasonable amount of time. The rupture of the metastable thin film is a thermally activated process, which proceeds through the opening of a tiny, molecular-sized hole that grows further under the action of surface tension. Since the presence of various detergents lowers the surface tension, thermal methods are not very effective for breaking oil-in-water emulsions in bilge water (Karlsruhe, G. H., Ed., “Emulsion Sciences, Basic Principles and an Overview”, Published by Physics and Astronomy, Springer, New York, N.Y., 2002). Chemicals used for breaking oil-in-water emulsions are sensitive to changes in the emulsion composition, limiting their effectiveness in treating bilge water (Colbert, J. C., Ed., “Foam and Emulsion Control Agents and Processes”, Published by Noyes Data Corporation, Park Ridge, N.J., 1981). Even if a robust demulsifier is available, it is generally undesirable to transport the material to sea. Electrolytic treatment systems have many undesirable attributes, including the need to maintain a stable pH, potential short circuiting, frequent electrode replacement, and the formation of an oil containing sludge that requires further treatment.
Accordingly, improvements are desirable in the treatment of emulsions, particularly oil-water emulsions.